1. Field of the Invention
Embodiments of the present invention relate to a radio frequency (RF) transmitter. More particularly, embodiments of the invention relate to an RF transmitter adapted to effectively compensate for output power variations due to the temperature and a process without adversely impacting power consumption.
This application claims priority to Korean Patent Application No. 2005-00564 filed Jan. 4, 2005, the subject matter of which is hereby incorporated by reference.
2. Description of the Related Art
Generally, the RF transmitter of a wireless communication system uses a high power amplifier (HPA) to transmit RF signals through an antenna. However, the output characteristics of the high power amplifier typically change in accordance with variations in temperature. Such temperature variations may be due to changes in the ambient operating temperature for the system, or the development of heat from internal components, such as semiconductor elements, with the high power amplifier. Generally speaking, if the temperature of the high power amplifier rises, its output power falls, but if the temperature of the high power amplifier falls, its output power rises. As a result of this phenomenon, the output power of a high power amplifier will dynamically change in relation to ambient operating temperature variations and/or the quantity of internally generated heat.
The output power of a high power amplifier may also change in relation to variations in the manufacturing process used to make the high power amplifier. That is, although high power amplifiers are manufactured according to a particular set of design specifications using defined manufacturing processes, non-uniform results may nonetheless be obtained due to uncontrolled deviations (or anomalous collective variations) in the processes used to fabricate the constituent semiconductor components of the high power amplifier, for example. Variations in the desired performance parameters between individual high power amplifiers result in variations in output power characteristics. Accordingly, output power variations for high power amplifiers must be conventionally compensated for in order to generate stable RF signals.
Decreased output power from the high power amplifier may degrade the communications link performance otherwise enabled by the RF transmitter. On the other hand, increased output power from the high power amplifier may cause interference with nearby devices.
In order to overcome such problems, an automatic level control loop has been used. The conventional automatic level control loop controls an output power of a power amplifier by detecting output power variations potentially caused by operating temperature fluctuations or manufacturing variances, and thereafter compensating for such variations. However, since the automatic level control loop directly controls the high power amplifier output power, and since the output power comprises RF band energy, a large amount of power may be consumed in the compensation process over the entire range of output power variations expected in many conventional power amplifiers.
Consider the example illustrated in FIG. 1 which is a block diagram of an RF transmitter including a conventional automatic level control loop.
As shown in FIG. 1, the RF transmitter comprises a digital-to-analog converting unit 110 for converting Inphase binary data to an Inphase analog signal, and converting Quadrature binary data to a Quadrature analog signal. The RF transmitter also comprises a low-pass filtering unit 120, a mixing unit 130 for modulating the Inphase analog signal and the Quadrature analog signal using an RF carrier wave, a signal multiplexer 140 for multiplexing the modulated Inphase signal and the modulated Quadrature signal to generate an RF signal, a power amplifier 150, an automatic level controller 160, and a reference current generator 170.
Digital-to-analog converting unit 110 converts Inphase binary data to an Inphase analog signal and converts Quadrature binary data to a Quadrature analog signal using a reference current Iref provided by reference current generator 170. Output levels of the Inphase analog signal and the Quadrature analog signal are determined by the amplitude of reference current Iref.
Low-pass filtering unit 120 filters the Inphase analog signal and the Quadrature analog signal to eliminate high frequency noise and to increase modulation efficiency. That is, low-pass filtering unit 120 generates an Inphase baseband signal and a Quadrature baseband signal by filtering the Inphase analog signal and the Quadrature analog signal.
Mixing unit 130 modulates each of the Inphase baseband signal and the Quadrature baseband signal using a carrier signal provided by a local oscillator circuit 190. Multiplexer 140 multiplexes the modulated Inphase signal and the modulated Quadrature signal to generate the RF signal. The transmitted RF signal is applied to power amplifier 150 for amplification.
Since the inherent power of the RF signal output from multiplexer 140, as applied to the transmitting antenna, is often insufficient to be close the desired wireless communications link at a defined signal-to-noise ratio, power amplifier 150 is used to boost the RF signal. Thus, power amplifier 150 receives a bias current (IPA—bias) and amplifies the power of the RF signal before it is applied to the antenna.
Automatic level controller 160 continuously controls the power of the RF signal. That is, automatic level controller 160 determines a difference value between a reference voltage (Vref) and an applied alternating current rectified voltage, and then supplies the bias current (IPA—bias) which is generated in accordance with the difference value. Applied bias current (IPA—bias) is thus a feedback signal to power amplifier 150 which adds or subtracts power from the RF output signal.
In this manner, power amplifier 150 and automatic level controller 160 form a feedback loop that maintains stable output power in the conventional RF transmitter. This feedback loop is often called an automatic level control loop. In the automatic level control loop, the output power of power amplifier 150 is fed back to automatic level controller 160. Automatic level controller 160 converts the output power of power amplifier 150 to a direct current signal by rectifying the output power voltage. This rectified output power voltage is then compared to reference voltage (Vref ) in order to determine a difference value (a voltage difference). This difference value, once converted to a corresponding current is used to modify (add to or subtract from) the current bias current (IPA—bias) applied to power amplifier 150. In other words, a feedback controlled bias current is supplied to power amplifier 150 in order to control the output of power amplifier 150. A conventional automatic level controller 160 will be explained in some additional detail with reference to FIG. 2.
FIG. 2 is a block diagram showing an automatic level controller 160 shown in FIG. 1. Referring to FIG. 2, the automatic level controller includes a reference circuit 161 controlling the reference voltage (Vref) such that it is stably provided in the presence of variable loading, and a rectifier 162 for rectifying an alternating current voltage which varies in the time domain and is output from power amplifier 150. The automatic level controller 160 also includes an adder 163 for determining a difference value between the reference voltage (Vref) and the rectified voltage received from rectifier 162. This difference value is applied to an integrator 164 which accumulates difference values as they vary in the time domain, and a voltage-to-current (V-I) converter 165.
Reference circuit 161 converts (or conditions) an externally supplied voltage to provide the defined reference voltage signal. Rectifier 162 rectifies the alternating current voltage output signal from power amplifier 150 into a direct current (DC) voltage in order to compare the converted DC voltage to the reference voltage. Adder 163 determines the difference value between the converted DC voltage and the reference voltage by subtracting the converted DC voltage from the reference voltage. Integrator 164 integrates the differences received from adder 163. V-I converter 165 generates the bias current (IPA—bias) corresponding to the difference value between the reference voltage and the output voltage, and applies it to power amplifier 150 in order to compensate for variations in output power.
As described above, when the output power of power amplifier 150 increases, power amplifier 150 will output a voltage greater than the reference voltage and adder 163 will subtract the output voltage from power amplifier 150 from the reference voltage (Vref ). Integrator 164 will intergrate the resulting negative difference values and output a corresponding integrated value to V-I converter 165. V-I converter 165 will then output a reduced operating current (IPA-bias) to power amplifier 150 as a bias current, thereby compensating for the increased output of power amplifier 150.
In the approach described above, the power amplifier operating within the RF domain is directly controlled in relation to variation in output signal. That is, the entire range of possible output power variations, whether due to operating temperature fluctuations or manufacturing process variances, is compensated by controlling only the power amplifier. Therefore, the conventional RF transmitter consumes a large amount of power during the compensation process which is dependent upon modifications to the bias current supplied to the power amplifier.